This invention relates generally to exhaust gas sensors, and specifically to exhaust oxygen sensors.
Oxygen sensors are used in a variety of applications that require qualitative and quantitative analysis of gases. Oxygen sensors have been used for many years in automotive vehicles to sense the presence of oxygen in exhaust gases.
For example, sensors have been used to sense when an exhaust gas content switches from rich to lean or lean to rich. In automotive applications, the direct relationship between oxygen concentration in the exhaust gas and the air-to-fuel ratios of the fuel mixture supplied to the engine allows the oxygen sensor to provide oxygen concentration measurements for determination of optimum combustion conditions, for maximization of fuel economy, and for the management of exhaust emissions.
A conventional stoichiometric oxygen sensor typically consists of an electrochemical pumping cell and a reference cell. Sensors conventionally used in automotive applications use a yttria-stabilized, zirconia-based electrochemical galvanic cell operating in potentiometric mode to detect the relative amounts of oxygen present in an automobile engine""s exhaust. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia ectrolyte, according to the Nernst equation:   E  =            (                        R          ⁢                      xe2x80x83                    ⁢          T                          4          ⁢          F                    )        ⁢          ln      ⁡              (                              P                          O              2                        ref                                P                          O              2                                      )            
where:
E=electromotive force
R=universal gas constant
F=Faraday constant
T=absolute temperature of the gas
PO2ref=oxygen partial pressure of the reference gas
PO2=oxygen partial pressure of the exhaust gas
Due to the large difference in oxygen partial pressures between fuel rich and fuel lean exhaust conditions, the electromotive force changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric oxygen sensors indicate qualitatively whether the engine is operating fuel rich or fuel lean, without quantifying the actual air to fuel ratio of the exhaust mixture.
Further control of engine combustion can be obtained using amperometric mode exhaust sensors, in which oxygen is electrochemically pumped through an electrochemical cell by an applied voltage. A gas diffusion-limiting barrier creates a current limited output, the level of which is proportional to the oxygen content of the exhaust gas. These sensors conventionally consist of two or more electrochemical cells; one of these cells operates in potentiometric mode and serves as a reference cell, while another operates in amperometric mode and serves as an oxygen-pumping cell. This type of sensor, known as a wide range, lambda, or linear air/fuel ratio sensor, provides information beyond whether the exhaust gas is qualitatively rich or lean: it can quantitatively measure the air/fuel ratio of the exhaust gas.
One example of a conventional configuration for a gas sensor with a pumping cell and reference cell is shown in FIG. 1 generally at 10. The pumping cell comprises an electrolyte 12 disposed between an outer electrode 14 and an inner electrode 16.
The reference cell comprises a solid electrolyte 20 disposed between the inner electrode 16 and a reference electrode 22. Layers 24 of dielectric material, such as alumina, are used as a substrate into which the cell components are placed. One or more backing layers 26 are disposed against the reference electrode 22. Generally, a ground plane (not shown) and a heater (not shown) are disposed between these backing layers 26.
Conventional sensors can also have only one cell with a clean air reference. With a one cell arrangement, however, the clean air reference requires cumbersome sealing and porting complexity, and presents additional elements that are prone to failure during operation.
What is needed in the art is a simplified means for sensing exhaust gas without the need for the complexity of two cells or a single cell with an air reference.
Disclosed herein is a device for sensing gas concentration in an exhaust flow comprising a dielectric substrate, a heater disposed within said substrate, a ground plane disposed within said substrate, and a cell consisting essentially of: an outer electrode disposed in electrical communication with an electrolyte, an inner electrode disposed in electrical communication with the electrolyte opposite to the outer electrode, and a protective layer disposed in fluid communication with the outer electrode, wherein the inner electrode is sealed such that gas contacting the inner electrode must first diffuse through the protective layer, the outer electrode, and the electrolyte.
The sensor provides a method for sensing the concentration of a gas in an exhaust stream, comprising: using a single cell disposed in a substrate, the cell consisting essentially of an outer electrode disposed in electrical communication with an electrolyte, an inner electrode disposed in electrical communication with the electrolyte opposite to the outer electrode, and a protective layer disposed in fluid communication with the outer electrode, wherein the inner electrode is sealed such that gas contacting the inner electrode must first diffuse through the protective layer, the outer electrode, and the electrolyte; applying a voltage to the cell, and measuring a current produced by the voltage, wherein the current is proportional to the concentration of the gas in the exhaust stream.